Now I am mud, now a leaf ... Leo Benedictus at the Acting Gym. Photograph: Murdo MacLeod

Performing on the Fringe is hard work, you know. Anyone who hopes to do justice to Underwater Macbeth for 25 nights on the trot has to be in really good shape - which is why the C Theatre Company have had the bright idea of setting up a daily Acting Gym. It's described as "the perfect warm-up to start your day"; this being Edinburgh, that means kick-off is at 12.30pm.

When I arrive at the venue, eight other participants are getting to know one another. Five are fringe actors, two are front-of-house staff ("just curious"), and one is a local woman, Margaret. Our coach, Georgina, starts us off with a general stretch and shake-down. After twisting and whirling my right arm as instructed, I discover that it has grown about an inch longer than the other one. Georgina assures us that this is perfectly normal.

Next comes a series of group exercises. We all stand in a circle and, acting purely on instinct, try to jump and land as one. It reminds me of Luke Skywalker's early Jedi training. After the jumping come the balls. We have to throw them to each other, calling out numbers to indicate who the catcher is. It is fun, and becomes quite hard when Georgina lobs an imaginary ball into the mix. Harder still is keeping myself from getting competitive about it.

From here, the exercises, which have names such as "body hoo-ha" and "shape, shape, reshape", begin to search for more instinctive and primeval feelings, until finally the group is stalking around the room, expressing the qualities of mud, leaves and lightning. Grimacing with embarrassment I find I am beginning to see the point. Yes, I realise, as all actors must, I am now definitely less anxious about making a tit of myself in public.